test: retry flaky pack/install test (#1316)
test: retry flaky pack/install test (#1316)

Retry the pack and install test, which has several moving parts and
fails once in a blue moon. Refactored such that tests are now independent,
i.e., npm pack and webpack can be run independently.

Fixes #1315
googleapis/google-auth-library-nodejs,googleapis/google-auth-library-nodejs